1. Field of the Invention
The invention relates to an apparatus for depositing a material layer originating from process gas on a substrate wafer, and to a method which uses said apparatus.
2. Background Art
The invention relates, in particular, to an apparatus for depositing a material layer by chemical vapor deposition (CVD), for example an apparatus for depositing an epitaxial layer on a substrate wafer composed of a semiconductor material such as silicon.
The basic construction of an apparatus for depositing a material layer originating from process gas on a substrate wafer is known and is evident, for example, from the description in WO 2007/050309 A1. Accordingly, such an apparatus comprises a reactor chamber, which is delimited by an upper dome, a lower dome and a side wall. Radiant heating systems are arranged above and below the reactor chamber and, during the deposition of a material film, generate enough heat in order that process gas directed over the substrate wafer is activated and a material layer emerging from constituents of the process gas forms on the surface of the substrate wafer. The substrate wafer is held by a susceptor surrounded by a preheating ring. The preheating ring lies on a liner, which is part of the side wall of the reactor chamber. It has the function of supporting the heating of process gas which is directed to the substrate wafer. Integrated into the side wall are feed and outlet openings for feeding in the process gas and for discharging waste gases emerging therefrom.
JP2006049503 A2 discusses an apparatus used for depositing an epitaxial film on a semiconductor wafer composed of silicon. The apparatus has the basic construction described above and furthermore has further feed and outlet openings integrated into the side wall of the reactor chamber. The further feed and outlet openings serve for feeding purging gas into that volume of the reactor chamber which is present below the susceptor, and for discharging the purging gas from this volume. In accordance with the description in JP2006049503 A2, gaseous compounds can pass through the gap between the preheating ring and the susceptor to the growing epitaxial layer and alter the resistivity of the epitaxial layer in the edge region of the semiconductor wafer. In order to prevent this “autodoping” effect, JP2006049503 A2 proposes covering the gap.
The inventors of the present invention have found that it is necessary to deal with certain problems when using an apparatus configured, in principle, like the apparatus described in WO 2007/050309 A1 or as described in JP2006049503 A2. This is because there is the risk that the radial profile of the resistivity of an epitaxially deposited layer composed of silicon, as considered over the diameter of the substrate wafer, will become markedly asymmetrical. Ideally, the profile is symmetrical or at least almost symmetrical. Furthermore, it is expected that particles will contaminate the deposited material layer to a comparatively high extent.